Bowden is a good idea, since you would not have to change subway lines. Actually, North Station on the green line trolley is the closest stop to the hostel.
Certainly you can a GPS it, but from Bowdon you'd be walking downhill (uphill is Beacon Hill) on a wide, sometimes windy street (New Chardon Street) toward the North End Italian neighborhood. On the left at the bottom of the street/hill, you would see a small parking lot with its trailer office. Just behind it are several parallel streets that run into Causeway Street (where the TDGarden is); pass Portland Street and Friend is the next one. Turn left. The street can sometimes look a bit sketchy (begging homeless folks), but is safe.
It's a good base from which to explore. Lots of sights within reasonable walking distance. Have fun.